The field of the invention is circuit test systems.
As the spacing/pitch between connection pads on circuit boards decreases, previously utilized testing mechanisms such as the xe2x80x9cbed of nailsxe2x80x9d text fixture used for the electrical test of bare circuit boards become inadequate. This inadequacy typically results, at least in part, from high cost, long delivery time, and/or insufficient density.
Although alternative testing mechanisms have been proposed, they too are at times inadequate or undesirable for testing high density/small pitch circuit boards. One such method is described in U.S. Pat. No. 4,843, 329 issued on Jun. 27, 1989 to Beha et al. (hereinafter xe2x80x9cBehaxe2x80x9d). Beha discusses a method for use in detecting whether two pads are electrically coupled (i.e. whether they are shorted together) by using an optical beam to generate a charge on a first pad (and any other pad connected to it), irradiating the board with an optical beam to cause an electron flux to be emitted from the second pad and a third pad, and detecting and comparing the flux from the second and third pads to determine if the second pad is electrically coupled to the first pad. This method is at times undesirable due to the need to cause photoemission of electrons and the need for testing within a vacuum chamber.
Another method is described in U.S. Pat. No. 5,357,194 issued on Oct. 18, 1994 to Ullman et al. (hereinafter xe2x80x9cUllmanxe2x80x9d). Ullman discusses using a series of contacts/probes coupled to a grid of light-transmissive electrically-conductive strips interconnected via photoconductive gates wherein a light source is utilized to selectively illuminate the gates to provide power to selected connectors. In essence, Ullman simply provides a test device comprising a grid of contacts/probes and a mechanisms for quickly selecting individual probes. As with the previous xe2x80x9cbed of nailsxe2x80x9d approach, the Ullman method can become problematic when testing high density circuit boards or boards on which the pads to not line up well with the probes. Although customized versions could be produced to match up with specific circuit geometries, use of such customized versions increases the cost and introduces a period of delay before the device is ready for use in testing a particular board.
Thus, there is a continuing need for new methods and devices for circuit board testing, particularly in regard to devices and methods suitable for low cost and timely testing of high density circuit boards.
The present invention is directed to methods and apparatus for fixture-less testing of circuits. More specifically, connectivity between any two conductive points on a circuit board is tested by coupling one conductive point to a voltage source to charge the conductor, de-coupling the voltage source, and coupling the second conductive point to a ground and measuring any current flow between the second conductive point and the ground to determine if the second point was charged via a connection to the first conductive point. The use of light activated bias boards comprising a photoconductive layer coated with a light-transmissive electrically-conductive layer may be advantageously used in coupling the conductors to the voltage source and ground.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.